Management of auricular transcutaneous neuromodulation and electro-acupuncture of the vagus nerve for chronic migraine: a systematic review

This systematic review study published in 2023 investigated the efficacy of auricular transcutaneous vagus nerve stimulation (at-VNS) and auricular electroacupuncture in the treatment of chronic migraine. Migraine is a neurologic condition that affects more than 1 billion people worldwide, causing intense pain, nausea, vomiting, and sensitivity to light and sound, representing the second most frequent cause of neurologic disability globally. The investigators conducted a systematic search across six databases through June 2022, initially identifying 1,117 publications. After a rigorous selection process based on specific criteria, nine randomized clinical studies were included in the final analysis, totaling 333 participants with a mean age of 34.8 years, of whom 41.7% were women.

The mean duration of migraine symptoms was 14.6 years, with a mean frequency of 10.8 attacks per month. The studies investigated different stimulation protocols, varying in frequency (1 to 25 Hz), session duration (8 minutes to 4 hours daily), and total number of treatments (single session to 12 weeks). The most commonly used application points were the auricular concha (cymba and cavum), in addition to specific points such as TE-19, ST-16, CO11, CO14, and GB-8. Methodological quality assessment using the PEDro scale revealed a mean score of 7.3 ± 0.8 points, with four studies achieving high methodological quality (≥5 points).

Risk of bias was assessed using the Cochrane ROB-2 tool, identifying only two studies with low risk. The main outcomes measured included pain intensity through the Visual Analog Scale (VAS), frequency and duration of migraine attacks, migraine-specific quality of life questionnaires (MSQ), depression (SDS) and anxiety (SAS) scales, and disability assessment (MIDAS) and headache impact (HIT-6). Six studies used functional magnetic resonance imaging (fMRI) to investigate changes in brain connectivity. The results suggest low-quality evidence for some positive effects of at-VNS at 1 Hz frequency compared with the control group post-treatment.

Specifically, there was a significant reduction in the frequency of migraine days and number of attacks in some studies. Neuroimaging analysis revealed alterations in brain areas related to pain processing, including the anterior cingulate cortex, periaqueductal gray, locus coeruleus, nucleus tractus solitarius, and frontal cortex. These neuroimaging findings provided evidence of the mechanism of action of auricular vagal stimulation, demonstrating connectivity between the peripheral branches of the vagus nerve and upper brainstem structures involved in pain modulation. Treatment safety was satisfactory, with only 7% of patients reporting adverse effects in one of the studies, leading to treatment discontinuation.

The remaining studies did not report significant adverse events. Limitations included heterogeneity in stimulation protocols, variability in follow-up periods, small sample size of individual studies, and risk of methodological bias in several studies. The inconsistency in stimulation parameters (frequency, intensity, duration) hindered direct comparison between studies and made it impossible to perform a meta-analysis. The clinical implications suggest that at-VNS and auricular electroacupuncture may represent safe and potentially effective therapeutic alternatives for patients with chronic migraine, especially considering the limitations and adverse effects of conventional pharmacologic treatments.

The non-invasive nature, low cost, and ease of application make these treatments attractive in clinical practice. However, the authors emphasize the need for future studies with greater methodological rigor, standardized protocols, and larger samples to establish definitive conclusions about the efficacy of these treatments in chronic migraine.